Field of the Invention
The present invention relates to a method of producing a laminated PTC thermistor which has a positive resistance-temperature characteristic and is useful in protection of overcurrent, control of temperature, degaussing, and so forth.
In recent years, the demand for plane-mounting of electronic parts has increased. Thus, laminated PTC thermistors have been required to be reduced in size and resistance. To meet such requirements, laminated PTC thermistors have been developed. In a laminated PTC thermistor, a ceramic piece is produced in which internal electrodes and semiconductor ceramic layers are alternately laminated to each other, and external electrodes are formed on both of the ends of the ceramic piece so as to be connected to the internal electrodes. With this structure, the thickness of the semiconductor ceramic layers and the internal electrodes can be reduced. Thus, the size of the electronic part can be reduced, and the plane-mounting can be easily performed. The resistance of the ceramic piece can be determined by adjustment of the distance between the internal electrodes. Thus, the dispersion of the resistances of electronic parts, caused by differences between the ceramic pieces thereof, can be eliminated, and the resistances of the electronic parts can be reduced.
Referring to a method of producing such a laminated PTC thermistor according to the related art, electro-conductive paste to form internal electrodes and ceramic green sheets to form semiconductor ceramic layers are alternately laminated to each other in such a manner that the conductive pastes for the internal electrodes are exposed onto one-side end of the ceramic green sheets and fired so as to be integrated with each other. Thereafter, electroconductive pastes to form external electrodes are applied onto both of the ends of the sintered ceramic piece and baked.
However, the following problems are caused: in the case where a base metal such as nickel or the like is used for the internal electrodes, the internal electrodes having a low melting point are ready to be oxidized when the internal electrodes and the semiconductor ceramic layers are laminated and fired to be integrated. Therefore, in the case in which a base metal such as nickel is used for the internal electrodes, the internal electrodes and the semiconductor ceramics are simultaneously fired in a reducing atmosphere so that the internal electrodes are prevented from being oxidized, and thereafter, heat-treated in the atmosphere or an oxidizing atmosphere below the firing temperature so that the PTC characteristic is realized. Thereafter, the external electrodes are formed.
However, the laminated PTC thermistor formed by the above-described method, when it is subjected to a conduction test at room temperature for 1000 hours, shows that the room temperature resistance is remarkably increased with the lapse of time. This time-dependent increase of the resistance, e.g., in an overcurrent protection circuit for which a low resistance is required, causes an increase of power consumption, an increase of a heating value, and the deterioration of the overcurrent protection function.